The present invention relates to distributed feedback (DFB) fiber lasers, and more particularly to an asymmetrical DFB fiber laser which operates in a single longitudinal mode, and a single polarization mode, and has a single sided output.
In a DFB fiber laser, a symmetrical Bragg grating is written into a length of rare-earth doped optical fiber to form a lasing cavity. Bragg gratings are created within the optical fiber by exposing the fiber to ultraviolet radiation to produce refractive index changes in the fiber. A periodic pattern is imposed on the impinging radiation by, e.g. superimposing a pair of beams of substantially monochromatic radiation to create an interference pattern. When the patterned radiation field impinges on the fiber, a corresponding pattern is imposed on the core of the fiber in the form of periodic fluctuations in the core refractive index. The general techniques for creating Bragg reflectors are now well known in the art. The Bragg grating fluctuations function as a wavelength selective reflector having a reflectance curve with a well-defined peak, thus allowing the fiber to output light of a fairly narrow wavelength band.
It is well known that a DFB fiber laser having a symmetrical Bragg grating (without a .pi./2 phase shift), and no end reflectors, will oscillate in two longitudinal modes spaced symmetrically around the Bragg wavelength. To obtain single frequency, i.e. single longitudinal mode, operation of a DFB fiber laser, an end reflector (mirror) can be used to change the round-trip phase shift in the cavity. The end reflector thus provides a single longitudinal mode, and also provides a single sided output that boosts output power in the direction of the output. This extra power is desirable in many circumstances. Alternatively, a phase shift of .pi./2 can be introduced into the grating by localized heating or UV light exposure of the fiber wherein the round trip phase condition is satisfied at the Bragg wavelength. The use of both the end reflector and introduction of the .pi./2 phase shift are discussed in U.S. Pat. No. 5,771,251 to Kringlebotn et al. While the reflector is effective for achieving operation in a single longitudinal mode and for producing a single sided output, the device still does not operate in a single polarization mode. Introduction of the .pi./2 phase shift produces a single longitudinal mode but does not provide single sided-output.
Polarization mode is also a crucial issue with DFB fibers lasers. Since single mode fibers usually have a circular cross section, there is no preference in gain and loss between the two Eigen polarization modes. This means that a DFB fiber laser normally emits two polarization modes. Fiber lasers such as this cannot be used in applications where single polarization light is required. A typical example in communications systems where the light from the fiber laser is coupled into a Lithium Niobate electro-optical modulator. Mode coupling mechanisms, such as introducing stress in the fiber, twisting the fiber, or stretching the fiber are the most current approaches to achieving single polarization operation of a DFB fiber laser. An article by Z. E. Harutjunian, et al titled "Single Polarization Twisted Distributed Feedback Fiber Laser", ELECTRONICS LETTERS, Vol. 32, No. 4 (Feb. 15, 1996) discusses in detail twisting of the fiber to achieve operation in a single polarization mode.
Accordingly, while there have been attempts to provide a DFB fiber laser which has one or two of the desired operating characteristics, none of the present solutions adequately provides a DFB fiber laser having simple structure which also has all three of the desired operating characteristics.